Bone mineral density assessment using mammography system

ABSTRACT

A method and system for determining the bone mineral density of a body extremity. An image of a body extremity is acquired using a mammography x-ray system whereby a bone mineral density can be performed on the image. The system for determining the bone mineral density of a body extremity includes: a support for supporting the body extremity; a detector for capturing an image of the body extremity; and an x-ray source adapted to project an x-ray beam through the body extremity toward the detector, the x-ray source having a voltage of no more than about 45 kVp and having a target/filter combination of rhodium/rhodium, molybdenum/molybdenum, molybdenum/rhodium, or tungsten/rhodium.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Continuation of U.S. Ser. No. 11/614,199 filed on Dec. 21,2006 entitled “BONE MINERAL DENSITY ASSESSMENT USING MAMMOGRAPHY SYSTEM”in the name of Huo et al., which claims priority to U.S. ProvisionalPatent Application No. 60/755,233, entitled “BONE MINERAL DENSITYASSESSMENT USING MAMMOGRAPHY SYSTEM”, provisionally filed on Dec. 30,2005 in the name of Huo et al., both of which are incorporated herein.

FIELD OF THE INVENTION

The invention relates generally to the field of mammography imagingsystem. More specifically, the invention relates to a system forassessing Radiation Absorptiometry (RA) based BMD (Bone Mineral Density)using a mammography x-ray imaging system.

BACKGROUND OF THE INVENTION

Osteoporosis is a skeletal disorder characterized by reduced bonestrength. It can result in increased risk to fractures, height loss,hunched backs, and pain. Bone strength is a function of bone mineraldensity (BMD) and bone quality. It is believed that bone mineral densitypeaks about the age of 30 for both men and women, and then declinesgradually. Some statistics have indicated that osteoporosis affectsapproximately 20 million people and is a cause of about 1.3 millionfracture incidents in the United States each year. As such, screeningfor bone mineral density is often desired.

Several common techniques have been used to measure bone mineraldensity, including bone puncture, radiation absorptiometry of singleenergy x-ray systems, DEXA (dual energy x-ray absorptiometry), andsonography.

Bone puncture can be an accurate but invasive procedure, which involvesthe extraction of bone mass from spine area. This procedure carriesrisk.

With regard to single energy x-ray systems, mineral loss in a person'sbones can be estimated from a single energy x-ray image of a body part.In diagnosing and treating bone diseases, it is common to takeradiographic images of the patient (e.g., skeletal features of thepatient), then either read the images directly or perform softwareanalysis on the images to extract information of interest. For example,in diagnosing or monitoring the treatment of osteoporosis, one mighttake x-ray images of selected skeletal bones, then perform computeranalysis on certain image features to determine bone volume, bonelength, bone geometric changes, bone strength conditions, bone age, bonecortical thickness, and bone mineral mass.

Typically for reading and interpreting radiographic images directly, thetreating physician will refer the patient to a radiologist, who cansupervise both taking the radiographic image and interpreting the imageto extract desired bone information, such as bone mass and bone contourirregularities. Alternatively, if the bone analysis is done, at leastpartially, by a computer analysis system, the x-ray images prepared bythe radiologist may be sent back to the treating physician's computersite or to another computer site for computer analysis.

DEXA is a device used by the hospitals to measure bone mineral density(BMD). In DEXA, two low-dosage x-ray beams with differing energy levelsare aimed at the patient's spine, hip or whole body using conventionalx-ray machines. The computer calculates the content of bone mineraldensity based on the relationship that different bones absorb differentenergy levels. Some consider DEXA to be accurate, but the apparatus isbulky and expensive and results in more radiation to the patients. U.S.Pat. No. 6,816,564 (Charles, Jr.) is directed to a technique forderiving tissue structure from multiple projection dual-energy x-rayabsorptiometry.

Sonography devices measure the bone mineral density of peripheral bones,such as heel, shin bone, and kneecap. But it is recognized that the bonemineral density in the spine or hip change faster than that in heel,shin bone, or kneecap. Thus sonography is considered by some to be notas accurate or sensitive as DEXA in the determination of bone mineraldensity. DEXA allows early detection of abnormal change in bone mass forits targets spine, hip, or whole body. However, sonography offersadvantages of lower cost and radiation-free.

U.S. Pat. No. 6,246,745 (Bi) describes a software system for determiningbone mineral density from radiographic images of a patient hand obtainedfrom conventional x-ray imaging system.

US Patent Application No. 2005/0059875 (Chung) describes a biosensor andmethod for bone mineral density measurement.

US Patent Application No. 2005/0031181 (Bi) is directed to a system andmethod for analyzing bone conditions using DICOM compliant boneradiographic images.

U.S. Pat. No. 5,712,892 (Weil), commonly assigned, is directed to anapparatus for measuring the bone mineral content of an extremity.

While such systems may have achieved certain degrees of success in theirparticular applications, there is a need for a system and method forbone mineral density screening, particularly wherein a medicalprofessional can readily and locally (e.g., at their office location)generate a bone mineral density report. A suitable system would be easyto use, reduced in cost, yet provide sufficient accuracy. Preferredwould be an on-site screening that can be utilized by physicians,radiologists, or other medical professionals.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an imaging system andmethod to acquire hand x-ray images suitable for bone mineral density(BMD) screening and analysis using a mammography x-ray imaging system.

Any objects provided are given only by way of illustrative example, andsuch objects may be exemplary of one or more embodiments of theinvention. Other desirable objectives and advantages inherently achievedby the disclosed invention may occur or become apparent to those skilledin the art. The invention is defined by the appended claims.

According to one aspect of the invention, there is provided amammography x-ray imaging system adapted to acquire hand images withsufficient image quality for the assessment of BMD by a computer-aidedsystem.

According to one aspect of the invention, the system includes an x-raygenerator, x-ray source/target, filtration, x-ray detector, and atemplate for positioning the hand.

According to another aspect of the present invention, there is provideda method of positioning the hand to obtain hand images with sufficientimage quality for bone mineral density assessment.

According to another aspect of the present invention, there is provideda preferred range of kVp (x-ray energy) and mAs (exposure) for a giventarget/filtration (built in a mammography x-ray image system)combination to obtain sufficient quality hand images when mammographyscreen/film systems are used as an image detector.

According to another aspect of the present invention, there is provideda method of converting analog images to digital images for computeranalysis. A film digitizer with a preferred dynamic range can beemployed to convert analog images to digital images for the computeranalysis.

An image of a body extremity is acquired using a mammography x-raysystem whereby a bone mineral density assessment can be performed on theimage. The system for determining the bone mineral density of a bodyextremity includes: a support for supporting the body extremity; adetector for capturing an image of the body extremity; and an x-raysource adapted to project an x-ray beam through the body extremitytoward the detector, the x-ray source having a voltage of no more thanabout 45 kVp and having a target/filter combination of rhodium/rhodium,molybdenum/molybdenum, molybdenum/rhodium, or tungsten/rhodium.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following more particulardescription of the embodiments of the invention, as illustrated in theaccompanying drawings. The elements of the drawings are not necessarilyto scale relative to each other.

FIG. 1 shows a curve illustrating the relationship between the x-rayexposure (the amount of x-rays reaching the screen/film) and the filmoptical density (OD) for a mammography system and a conventional x-rayimaging system.

FIG. 2 shows a curve illustrating the difference in contrast betweenconventional systems and mammography systems.

FIG. 3 illustrates the system in accordance with the present inventionfor obtaining hand images for BMD assessment using a mammography system.

FIG. 4 illustrates the system in accordance with the present inventionwherein filtration is added proximate the x-ray source.

FIG. 5 illustrates the system in accordance with the present inventionwherein filtration is added proximate the support supporting the bodyextremity.

FIG. 6 illustrates the system in accordance with the present inventionwherein filtration is added proximate the detector.

DETAILED DESCRIPTION OF THE INVENTION

The following is a detailed description of the preferred embodiments ofthe invention, reference being made to the drawings in which the samereference numerals identify the same elements of structure in each ofthe several figures.

It is noted that the American Cancer Society recommends that women overthe age of 40 years obtain annual mammograms. Millions of women havetheir annual screening mammograms each year at hospitals or breastimaging centers. Accordingly, Applicants have noted it would bedesirable for women to have both their annual mammography screening anda bone mineral density screening done in one visit, at one location, andusing one imaging system.

Conventionally, extremities (e.g., hands and feet) are imaged usingconventional x-ray system, which generates an x-ray beam adapted tocapture both low and high-density objects (i.e., bone and soft tissue)on a detector (film or digital) that are designed with a wide dynamicrange.

In contrast, mammography imaging systems are configured for highcontrast (i.e., narrow dynamic range) to image the soft tissue in thebreast for the purpose of detection and diagnosis of breast cancer.

The present invention is directed to a system and method for acquiringhand images using a mammography x-ray imaging system. A treatingphysician or a computer-aided system can then analyze the acquiredimages for bone mineral density (BMD) loss assessment. It is intendedthat the use of a mammography imaging system to acquire hand images forassessing BMD can improve the workflow and access for women to BMDexams, so as to reduce the cost and improve the efficiency of screening.

Conventional x-ray imaging systems have been employed to image varioushuman body parts (e.g., head, neck, chest, abdominal, and extremities)to detect and diagnose various diseases. Because of the bone structuresand thick body part, high-energy x-ray is required to provide sufficientpenetration. Also, a wide range of x-ray energies (for example, from 50kVp-140 kVp, dependent on the selection of kVp) is available to providea suitable x-ray photon energy level (kVps) when imaging different bodyparts. Tungsten targets are typically used in conventional systems tomeet the needs of a wide-range of high energy x-rays. Generally, thickerand/or denser body parts require higher x-ray energy to providesufficient penetration to achieve desired image quality while keepingthe patient dose at minimum.

For example, 50-60 kVps are typically employed for extremities, 70-90kVp for hip and skull, and 100-130 kVp for chest. The signal strength(i.e., the amount of x-rays) reaching the detector can vary for a givenx-ray energy. The weakest image signals are typically behind or withinthe dense and thick body parts (i.e., high attenuation), such as bone orabdomen. The strongest image signals reaching the detector are in thearea of cavities or thin body part (i.e., low attenuation), such as theclear lung area or low-density soft tissue.

However, there is a non-linear relationship between x-ray exposure(i.e., amount of x-rays) to the x-ray film and the film optical density(OD). Thus, neither underexposure (e.g., not enough penetration) oroverexposure (e.g., too much x-rays penetrated the film) are desirable.

FIG. 1 illustrates the relationship between the x-ray exposure (theamount of x-rays reaching the screen/film) and the film optical density(OD). The curve is usually called the H&D curve which characterize theuniqueness of a screen/film system in its response to x-ray exposure.Two curves are shown in FIG. 1: one for a screen/film mammography systemand one for a conventional screen/film x-ray imaging system. The twocurves illustrate a difference in latitude 102 between the conventionaland mammography screen/film systems, referenced as 102-C and 102-M,respectively. More particularly, FIG. 1 shows characteristics curves fora mammography screen/film system (e.g., high contrast, narrow latitude)and a conventional screen/film system (e.g., low contrast, widelatitude).

Latitude is defined as the wide linear range of the optical density overthe exposure. That is, latitude refers to the range of relative exposurethat will produce optical density within the accepted range fordetection and diagnosis.

Information captured on the shoulder and beyond is referred to asoverexposed 104, while information captured on the toe is referred to asunderexposed 103. The wider latitude in conventional screen/film reducesthe likelihood of overexposure or underexposure of films on the shoulderand toe. Such overexposure and underexposure are considered to beundesirable image quality. When undesirable images occur, a retake ofthe image is required to capture sufficient information for detectionand diagnosis. Underexposure of the conventional and mammographyscreen/film systems is shown in FIG. 1 as 103-C and 103-M, respectively.Overexposure of the conventional and mammography screen/film systems isshown in FIG. 1 as 104-C and 104-M, respectively.

An x-ray film digitizer can be employed to convert an analog x-ray imageto a digital image. Such x-ray film digitizers are well known.

Still referring to FIG. 1, only the signals (exposures) within a limitedrange (latitude 102) are visible or recognizable by an x-ray filmdigitizer. Because of the wide range of densities in the body parts, therange of image signals and their strength which reach the film isconsiderably wide. Therefore, conventional x-ray films are design toprovide wide enough latitude 102 to properly capture a wide range ofimage signals on the film.

Mammography systems are designed to capture x-ray breast images,particularly for the detection and diagnosis of breast cancer. Since theattenuation or density differences in the different parts of breasttissues are small, mammography systems employ x-ray equipment anddetectors specially designed to optimize breast cancer detection. Usinglow x-ray photon energies generally will provide better differentialattenuation between the soft tissues than using higher energy x-rayphotons (50 kVp and above). However, low x-ray energy has a highabsorption and therefore delivers a relatively high dose.

Screen/film systems used in mammography are also designed to maximizethe contrast for the captured image signals and require a certain ofamount of radiation to ensure sufficient image quality for the cancerdetection task.

Referring to FIG. 2 there is shown the H&D curves of a screen/filmsystem for mammography and a screen/film system for conventional x-rayimaging system, illustrating the difference in contrast 201 between theconventional and mammography screen/film systems, referenced as 201-Cand 201-M, respectively.

More particularly, FIG. 2 shows characteristic curves for a mammographyscreen/film system (e.g., high contrast, narrow latitude) and aconventional screen/film system (e.g., low contrast, wide latitude). Thegraph illustrates the difference in film contrast between the two typesof screen/film systems for an object with the same object contrast. FIG.2 was obtained by projecting object 1 (a portion of aluminum step wedge)within the proper exposure range for the mammography system, while theentire step wedge was captured within the wide dynamic latitude of theconventional screen/film system.

Minimizing the dose while providing sufficient image quality to enhancethe low contrast detection imposes extreme requirements on mammographicequipment and detectors. Because of the risk of ionizing radiation, someprefer to minimize the dose and optimize the image quality. Theseconcerns have led to the refinement of dedicated x-ray equipment,specialized x-ray tubes, compression devices, and/or optimized detectorsystems. The imaging requirements impact the design of the x-ray tube,peripheral mammographic equipments, and film/screen detectors.

X-ray tubes designed specially for mammography provide can provide anearly optimal x-ray spectrum for a good subject contrast of softtissues while maintaining a radiation dose as low as possible.

Some experiments have shown that for a tissue thickness having a 3-6 cmrange (e.g., typical compressed breast thickness), preferred x-rayenergies are typically generated through a molybdenum target with a kVprange between 24 to 32 kVp.

A maximum tube voltage of the x-ray source for mammography isapproximately 45 kVp.

The x-ray source is defined by a target and filter combination(sometimes referred to as target/filter). Examples of target/filtercombinations includes molybdenum/molybdenum, molybdenum/rhodium,rhodium/rhodium, and tungsten/rhodium.

The contradictory requirements of high subject contrast and lowradiation dose are difficult to accomplish, and indicate mono-energeticx-rays as the best choice. However, x-ray energy of the x-ray tube ispoly-energetic.

Referring to FIG. 3, there is shown a system in accordance with thepresent invention for obtaining hand images for BMD assessment using amammography system. A source 301 emits x-ray energy 304 directed towardan object to be imaged (shown as fingers/hand in FIG. 3). A handtemplate 307 can be provided to properly position the object. One ormore phantom/calibration/step wedges 308 can be positioned proximate theobject made of a material (e.g., aluminum) to approximate the densityvariations of a human extremity. Template 307 and wedge 308 can bepositioned by frame 306.

Referring to FIG. 3, built-in filtration 302 is added to the x-ray tubeto remove some of the low energy x-rays as the low energies contributeto tissue dose without contributing significantly to image formation.The highest x-ray energies in the x-ray beam are a function of the peakoperating voltage (kVp) applied and added filtration at the x-ray tubeport. A fixed amount of filtration (for example, molybdenum/rhodium) isbuilt in the system to remove the low-energy x-rays. A rotating anodedesign is used for some mammographic x-ray tubes. Molybdenum targets arecommon, although tungsten is used in many tubes. A dual trackmolybdenum/rhodium target and molybdenum/rhodium filtration is used by amanufacturer.

A combination of target and filtration determines the x-ray beamquality, which in turn determines the image quality (contrast andexposure level).

The x-ray beam quality determined by the combination of the target andfiltration in conjunction with the x-ray energy range (operating kVprange) provide a distinction between mammography x-ray systems andconventional x-ray systems, and this determines the difference in theirapplications.

With regard to the image detector, the beam quality determines how muchsignals can penetrate through different body parts with differentthickness, reaching the detector. The characteristics of a detector(FIG. 1 and FIG. 2) determine the way in which the signals are capturedand presented in an analog and/or digital format by the detector.Detectors are customized to the need for conventional x-ray systems andfor mammography x-ray systems. As was shown in FIG. 1, a conventionalscreen/film system has a wider latitude 102-C to capture the wide rangeof signals in strength for imaging head, neck, chest, abdominal andextremities.

Hands can be imaged using conventional x-ray machines. The high x-rayenergy in conventional x-ray machine provides sufficient penetration tosee the details in the bone while the wide latitude of the screen/filmsystem allows capture of both the soft tissues (low attenuation) andbone (high attenuation) in one image. As was shown in FIG. 1,information captured on the shoulder and beyond is referred to asoverexposed (104-C and 104-M), while information captured on the toe isreferred to as underexposed (103-C and 103-M). Both overexposure andunderexposure are not desirable. The wider latitude in conventionalscreen/film reduces the likelihood of overexposure or underexposure offilms on the shoulder and toe.

Mammography screen/film systems have a higher contrast than conventionalscreen/film systems. This was illustrated in FIG. 2, where thedifference in optical density on the vertical axis is used to measurethe film. For example, an aluminum step wedge is an object havingvarying attenuation. The thicker the step wedge, the more x-rays areattenuated, and accordingly, the less x-rays reach the film. For a givenobject contrast (difference in exposure along horizontal axis in FIG.2), the film contrast 201 for mammography screen/film is higher thanthat for conventional screen/film system. While the high contrast inmammography screen/film detectors assists to signify the smalldifference among the soft tissues in a breast, the narrow latitude 102allows it to capture the information from a portion of the aluminum stepwedge. It is difficult to capture both soft tissues and high-densitybone structure on one single image. One can increase or decrease thex-ray energy and x-ray intensity to project the desired portion of thealuminum step wedge into the narrow latitude. For example, one canincrease the x-ray energy to increase the exposure to the film under thethicker part of the aluminum, so that the thicker part of the aluminumgot exposed properly within the narrow latitude. Note that when thethicker part of the aluminum gets exposed properly, the portion ofaluminum at the thin end likely gets overexposed. This indicates thatoverexposure of soft tissue or underexposure of bone is likely to occurwhen a mammography screen/film detector is used.

It is well known to use a cassette to hold a screen/film for x-rayimaging. If the screen/film employs a phosphor storage phosphor material(such as used for computed radiography), the storage phosphorscreen/film can be disposed within a cassette for imaging. FIG. 3generally shows a cassette 310 which can house computed radiographyplate 311 (having a storage phosphor layer). Cassettes designed formammography can include some particular attributes. Some cassettes aremade of a low attenuation carbon fiber and/or have a single highdefinition screen used with a single emulation film.

Because of the difference in the requirement for the x-ray beam qualitybetween mammography and conventional x-ray image systems, and in therequirement for characteristics of screen-film detectors between the twosystems, it is difficult to image hands using a low-energy (i.e.,mammography) x-ray beam quality system to get sufficient penetration ofthe bone while not overexposing the soft tissues in the hand.

For an accurate assessment of BMD using hand x-ray images, a sufficientamount of soft tissue needs to be visible as well as the detailed bonetrabecular structures. Overexposing the soft tissue or under exposingthe bone structure on the films can easily occur if proper techniquesare not used to image the hands. These techniques include the rightchoice of target/filtration combination, additional filtrationin-between the target to imaged object (hand), additional attenuationmaterial in-between imaged object to the image receiver, speed andlatitude of screen/film system.

Since mammography is particularly designed for breast imaging, thelimits or constraint set on how the system can be used does limit thechoices of the kVp and mAs combinations along with other factors tocapture the right exposure for hands. For example, 1) the maximum x-rayenergy from mammography system is set at 45 kVp for rhodium target and35 kVp for molybdenum target. This constriction limits the choices touse higher energy x-rays to get a good penetration of bone. 2) A lowlimit set on the output (x-ray intensity) for mammography x-ray units israther high (4 or 5 mAs), it often causes overexposure to the softtissue on hand image. 3) The distance from the source (target) to imagereceiver on mammography system is often fixed (65 cm), so the x-rayradiation cannot be lowered by increasing the distance, which is a wayoften used in conventional x-ray system to lower intensity of the x-rayradiation reach the film. A suggested distance between target and imagereceiver for imaging hands is 40 inches.

As mentioned above, a high kVp is employed to obtain a good penetrationof the hand. However, when kVp increases for a selected mAs setting, theradiation output (exposure to the film) increases as more x-rays areable to penetrate the object. To avoid overexposure of the soft tissue,it is desired to reduce the amount of the exposure to the film byreducing the mAs and increasing the distance from the source to thedetector. As a result of these limitations on the high-end kVp, minimummAs and the fixed distance from source to receiver on the mammographysystems, when good penetration is obtained, soft tissue is oftenoverexposed. Conversely, when soft tissue is appropriately imaged,sufficient penetration of bone cannot be reached. These limitationsaggregated the problem in finding the right techniques to appropriatelyimage the hand using mammography x-ray image systems and/or mammographyscreen/film detectors

To address the various problems discussed above, Applicants havereplaced the mammography screen/film detectors with digital detectors,that is, direct digital radiography (DR) and computed radiography (CR)designed for mammography. These detectors generally have a wide dynamicrange of the pixel value over the exposure levels. With a combination ofhigh x-ray energy (kVps) and mAs on the mammography machine, a suitableimage quality for both soft tissue and bone detail has been obtained.Note that cassettes for mammography can be used to hold a CR screen whenCR detectors are used.

Further, the mammography screen/film system was replaced with low speed(<150) conventional screen/film detectors. For example, film designedfor general radiography, such as Kodak X-sight G/RA, X-sight L/RA film,TMAT G/RA film or TMAT L/RA film or Insight film family. Thesedetectors, as mentioned above, have wider latitude than mammographyscreen/film system. The screen/film were placed into a mammographycassette and positioned for imaging (for example, inserted into abucky).

Using the x-ray energy of a mammography system and the conventionalscreen/film system allowed Applicant to generate hand images withsufficient good image quality for BMD assessment.

Further, mammography films can be replaced with conventional radiographyfilms (with speed of slower than 400). Thus the combination of amammography screen and a conventional radiography film was used toacquire hand images.

Further, the screen/film configuration in terms of its position relativeto incoming x-rays direction was investigated. In mammography, a singleback screen configuration (i.e., placing the film between the x-raysource and the screen) is often used to maximize the image sharpness andthe efficiency of the screen in converting absorbed x-ray energy tolight. For imaging a hand using mammography, a single front screenconfiguration can be used to reduce the x-ray to light conversionefficiency of the screen to avoid overexposure to the hand tissue. Thatis, the screen is placed between the x-ray source and the film. Examplesof a mammography screen/film system which can be used in the frontscreen configuration are Kodak MinR screen or MinR 2000 screen or MinR2190 screen or MinR 2250 screen or MinR EV screens with MinR-L film orMinR 2000 film or MinR EV film. Screens designed for conventionalradiography such as Lanex fine screen, Lanex medium screen, Lanexregular, Lanex Fast (Lanex screen family) or Insight screen family canreplace the screen designed for mammography in either a back screen orfront screen configuration.

Some images acquired in the configurations described above have beenreviewed using an available computer-aided BMD assessment system. (Notethe computer-aided assessment system was originally designed to assessthe BMD using the hand images acquired from conventional x-ray machineswith the conventional screen-film system (U.S. Pat. No. 6,246,745)). Thecomputer-aided assessment system relates to input images acquired usingx-ray energies of 50 kVp and above and background optical density (OD)on the film of 1.0−/+0.1. The background OD obtained from the aboveconfiguration is higher than 1.0 because the constraints set on kVp,mAs, distance from the source to detector and the choice of thetarget/filtration combination. From the results, Applicants believe thatlow energy x-ray beams from mammography x-ray systems when combined witha detector with sufficient wide latitude in its H&D curve can generatehand images with adequate image quality for BMD assessment.

Applicants investigated using mammography screen/film systems asdetectors to capture hand images for BMD assessment recognizing thatthere are limitations on the mammography x-ray imaging systems and theproperty of mammography screen/film designed breast imaging, and thatthere are numerous combinations of multiple variables (choice oftarget/filtration, added material for additional filtration, kVps, mAs,screen/film combinations). Mammography screen/film system werecategorized into two categories, high and low contrast. Hand phantomimages with sufficient penetration of bone and sufficient soft tissuerequired by the software can be achieved with selections oftarget/filtration combination, kVp and mAs, positioning of hands,additional filtration, and types of screen/film.

At 4 mAs setting when the images were acquired using a high-contrastscreen/film (Kodak MinR EV system), the acceptable kVp range (whichgenerates acceptable image quality for BMD assessment) is between 31 and35 for molybdenum/molybdenum target-filtration combination, is between30 and 34 for molybdenum/rhodium combination target-filtrationcombination, and is between 29 and 32 for rhodium/rhodium combinationtarget-filtration combination.

When increasing mAs setting from 4 mAs, the acceptable kVp range wasshifted to a lower kVp range. Both the lower and upper bound kVp valuesreduced a rate of 1 or 2 kVp per mAs. This results since the radiationexposure increases as the mAs increases at a given x-ray energy setting(kVp). An acceptable kVp range may get smaller at a higher energy asthere is minimum kVp setting for each target/filtration combination. Theminimum kVp for molybdenum/rhodium is 24. The minimum kVp forrhodium/rhodium is 25.

When a low-contrast mammography screen/film system was used (e.g., KodakMinR L), the acceptable kVp range at each corresponding mAs setting iswider than that for the high contrast screen/film system. However, theminimum acceptable kVp at each mAs setting is similar to that of thehigh-contrast screen/film system. Thus minimum x-ray energy (kVp) isrequired, regardless of the type of screen/film combination, to have thepenetration of bone required for BDM analysis.

Further, additional filtration material can be placed between the handand the x-ray source target to reduce the amount of x-ray radiation,thus avoiding overexposure for the soft tissue, especially at higherkVps.

While this can reduce the output to avoid overexposure of soft tissue byadding filtration additional to the built-in filtration, this canfurther increase the penetration of the bone by using higher energyx-rays. A wider range of kVps can be employed to generate the handimages with sufficient image quality, thus increasing the robustness ofits implementation. It is known that different mammography x-ray unitsare calibrated differently. In addition, the thickness and size of handscan vary. A wider range of kVps can increase the robustness of thesystem to generate images for accurate assessment of BMD by the computersystem.

The additional filtration materials can be aluminum of thickness between0.02 to 12 mm, polymethyl methacrylate of thickness between 0.5 to 120mm, copper 0.001 to 0.4 mm or other material with the thickness rangesthat can provide the x-ray intensity attenuation from 5% to 99.9% of thex-ray intensity without additional filter material.

Further, additional material can also be placed between the hand and thescreen film system to further attenuate the amount of x-rays to avoidoverexposures to the film. This can also be achieved by using thicker orhigh attenuation material for the front cover of the screen filmcassette.

Use of grid in the Bucky can be applied to reduce the x-ray exposure tothe film.

When hand images are acquired using conventional x-ray units along witha conventional screen/film system, a desktop scanner can be employed.For images acquired using mammography x-ray units, a x-ray filmdigitizer with a wide dynamic latitude can be used to read the widerange of information captured on the mammography film. The sufficientdynamic latitude is defined as the Dmin (minimum optical density) andDmax (maximum optical density) recognizable by the digitizer. Dmin of0.2 and Dmax of 4.0 have been recommended to capture the detailsrequired for the computer analysis.

When acquiring the images, the patient stands on the chest wall side ofthe mammography system facing the gantry. One hand is positioned on atemplate with an aluminum step wedge placed proximate the thumb and theindex finger (for example, refer to U.S. Pat. No. 6,246,745 (Bi)). Thehand is positioned to lay flat on the template.

As shown in FIGS. 3 and 5, frame 306 with the hand template 307 placedat the bottom and the additional filtration material placed on the topcan be used to position the hand when additional filtration is requiredto get adequate image quality.

Although, the added materials for additional filtration 305 is preferredto be placed in between anode/target and imaged object, another way toadd filtration is to place the material on the breast compression plateattached to the mammography x-ray unit or simply use the compressionplates. Another way is to mount the filter material on a support thatcan be attached to the mammography machine the same way as thecompression plate does. Another way is to attach the add-on filtration303 on the x-ray exit window of the x-ray tube.

With reference to FIG. 4, additional filtration 303 can be addedproximate x-ray source 301. Alternatively, as shown in FIG. 5,additional filtration 305 can be added proximate frame 306. Stillfurther, as shown in FIG. 6, additional filtration can be added tocassette 310. Combinations of these can also be employed. For example,FIG. 3 shows the use of added filtration 303, 305, and 309.

The acquired hand images can be evaluated by a computer or a treatingphysician for BMD assessment purpose. The image quality can be evaluatedby a computer system. The system uses a step wedge for a calibrationpurpose. The system can employ a test procedure to assess if sufficientimage quality (sufficient soft tissue and adequate penetration of bone)is achieved. Other systems may have different requirements in imagequality. However, the calibration for adequate penetration and/orsufficient soft tissue ensures the accurate assessment of BMD. Thepresent invention provides a method to acquire digital hand images usingmammography x-ray system for the purpose of BMD assessment.

Thus, the present invention provides a method of acquiring hand x-rayimages. According to one aspect of the present invention, the methodcomprises the steps of generating a digital or analog x-ray radiographof human hand using mammography imaging system. A cassette and screen isused wherein having a MO/MO or MO/Rh target/filter combination (MO beingmolybdenum, and Rh being rhodium) with exposure level lower than 10 mAs.In one arrangement, a phosphor screen/film combination is inserted intoa cassette to hold screen and film for x-ray imaging.

According to another aspect of the present invention, there is provideda system to acquire hand images on a radiograph using a mammographyx-ray system. The system includes x-ray generator with rotating targetsand filtration materials designed for mammography, image receivers andadditional filtration.

1. A radiographic imaging system for capturing an image of a bodyextremity, the image being suitable for determining a bone mineraldensity, the system comprising: a support for supporting the bodyextremity; an x-ray source configured to project an x-ray beam throughthe body extremity, the x-ray source having a voltage of no more than 45kVp and having a target/filter combination of rhodium/rhodium,molybdenum/molybdenum, molybdenum/rhodium, or tungsten/rhodium; adetector configured to receive the x-ray beam after the x-ray beampasses through the body extremity and to capture an image of the bodyextremity; and an attenuation filter separate from the target/filtercombination, the attenuation filter being positioned between the x-raysource and the detector and between the body extremity and the detector,the attenuation filter being configured to attenuate a low-energyportion of the x-ray beam that has passed through the body extremity. 2.The radiographic imaging system of claim 1, wherein the detector is acassette housing conventional general radiography film.
 3. Theradiographic imaging system of claim 1, wherein the detector is acassette housing mammography radiography film.
 4. The radiographicimaging system of claim 1, wherein the detector comprises a computedradiography plate comprising a storage phosphor material.
 5. Theradiographic imaging system of claim 1, wherein the detector is a directdigital radiography (DR) detector.
 6. The radiographic imaging system of1, wherein the attenuation filter is selected from the group consistingof: aluminum having a thickness of between 0.02 to 12 mm; polymethylmethacrylate having a thickness between 0.5 to 120 mm; and copper havinga thickness between 0.001 to 0.4 mm.
 7. The radiographic imaging systemof claim 1, wherein the attenuation filter is comprised of a material ofa thickness range which provides x-ray intensity attenuation from 5% to99.9% of the x-ray intensity without the attenuation filter.